This invention relates to induction furnaces and, more specifically, to vacuum induction furnaces.
Many attempts have been made to construct a long-lived vacuum induction furnace which have generally proved to be unsuccessful. In the typical furnace, a crucible is partially surrounded by a pressure vessel and the remainder of the crucible is surrounded by a coil assembly which is sealed to the pressure vessel. Typically, the coil assembly is defined by a plurality of spaced, electrically conducting coils which are provided with coolant passages and insulators are interposed between adjacent convolutions of the coils. The resulting interior surface of the coil assembly which confronts the crucible will have many irregularities therein due to inability to obtain precise alignment of the coils about a desired axis and due to irregularities in the surfaces of the components, particularly the insulators. Because the crucibles typically used in such furnaces are somewhat porous, the application of a vacuum to the interior of the vessel will tend to pull air through the crucible from the vicinity of the coil assembly. And, because the coil assembly is formed of multiple coil convolutions and multiple insulators, air can pass through the coil to ultimately be drawn into the crucible. Consequently, either extraordinarily large vacuum pumps have been required or desirable high vacuums cannot be obtained.
It has been proposed to solve the problem by sealing the interior surface of the coil assembly by applying thereto a fiber-reinforced rubber which is vulcanized to the interior surface of the coil. While such a seal will perform its intended function for a short period of time, longevity of seal life is well below that desired.
In particular, when the vacuum is applied to the interior of the vessel, it will tend to draw the seal towards the crucible. Because the interior surface of the coil assembly to which the seal is vulcanized has many irregularities therein, poor bonding is present and, as a result, the seal will be drawn away from the interior surface of the coil assembly and spaced therefrom. The resulting lack of intimate contact with the coil assembly precludes cooling of the seal by the coolant carried in the coolant conduits in the coils with the consequence that the seal overheats and soon fails.